Hi, I felt the same way when I first got my RV. Luckily I will soon live in a rural area of TX where we solve problems like that locally
but its very low crime for that reason.
I thought about bars as well, but if you put bars then criminals will wonder what you have inside that you need bars for and well you know where that leads. I also thought about replacing the door with a better model, until I found out how expensive they are. Then I thought about strengthening the door on the inside, but then you end up putting too much weight on cheap hinges screwed into insufficient structure, and that would mean that just doing alittle bit would turn into a huge job.
I finally just ended up puting a slide chain (like all homes have) from the door to the inside wall where I found a stud. I had to find shorter screws because the door is not that thick, but it seems to do the job in that at least it will make alot of noise if someone tries to get in.
I also used to close my handrail over the door when I was gone not for making anything secure but just a bad habbit, then I realized I was just advertising that noone was home.
The only other thing I did to the door (for energy savings) was to remove all the molding around the door on the inside and foam inside there. It makes a huge difference as there is a huge gap all the way around the door frame, just be carefull that molding is very easy to break. But I put up new molding from Lowes anyway for a fresh new look
So just realize that you are in an RV and there are some things that an RV can't be, then go get you a slide chain and install it...
This is all assuming that your door is in good shape to start with.